1. Field of the Invention
The field of art to which this invention relates is optical data storage. Specifically, this invention relates to optical data storage based on holographic recording within a rotatable spherical medium, preferably contained within a fluid.
2. Description of the Related Art
The inherent theoretical capacity of a holographic storage medium in bits is V/.lambda..sup.3 (according to a calculation by P. J. Van Heerden (1963) Appl. Opt. 2 (4): 393), where V is the volume of holographic material and .lambda. is the wavelength of light inside the material. This leads to very high potential capacities and is a major reason for the interest in holographic storage. For example, a 1 inch cube with a refractive index of 1.5 has a theoretical capacity of 4.4.times.10.sup.14 bits at 500 nm wavelength, or 55 terabytes. In order to approach this value in practice, page sizes should be as large as possible (in the range of 10.sup.6 to 10.sup.8 bits), and the maximum number of pages should be stored that the medium can support. Since a page is written and read out in parallel, data rates are also inherently high and typically limited by the speed at which data can be loaded onto the page-composing device (during writing) or down-loaded from a camera (during readout). For example, a page size of 4.times.10.sup.7 bits need only be read out once every second to be comparable to existing magnetic hard disk readout rates of 5 MB/s.
Typically the holographic medium in previously described holographic storage systems is formed in the shape of a cube or cuboid, which is fixed in position. Also, the optical beam bearing the data image is typically incident from a fixed direction. A single hologram is recorded by allowing an image beam containing a two-dimensional "page" of data to be incident on the medium simultaneously with a simple "reference" beam derived from the same source, but incident at a different angle. The two beams form a stationary optical interference pattern, which is recorded by the holographic medium in the form of a refractive index image which follows the interference pattern. Reconstruction of the data image is achieved by illuminating the holographic medium with a single beam identical to the reference beam, causing bragg diffraction to reproduce the original data image beam.
Illumination with a beam identical to the reference beam in every respect except its direction results in negligible reconstruction of that data page. The angular range of the reference beam directions around that of the original which may be used and still cause readout of only the single desired data page is a function of the physical parameters of the systems, but notably of the angle between the reference and data image beams. This angular proximity is referred to as the "angular bragg width." This angular selectivity allows recording of multiple holograms of data pages by changing the angle of incidence of the simple reference beam.
The angular multiplexing technique described above has the disadvantage that some means must be used to vary the reference beam direction, such as a galvo mirror or electro-optic or acousto-optic means. These techniques are difficult to realize in practice and have difficulty in achieving a wide range of angles. Also, since the angle between the reference and data beams changes between pages, the angular separation of holograms is not constant throughout. These features lead to a reduction in the achievable capacity of the storage system.
Use of a rotatable recording medium and fixed beams can alleviate these limiting features. A cylindrical medium is disclosed in U.S. Pat. No. 4,017,144, to Staebler. In the case of the cylinder, simple corrections in the beam aberrations are not possible in the presence of a refractive index change after recording. Pixel beams diffracted horizontally (so their axes exit the cylinder surface at normal incidence) have aberrations which are easily corrected, but pixel beams diffracted vertically (with a component along the cylinder axis) have their directions changed as they refract at the surface, causing them to land in the wrong position at the detector, and to have their best focus plane in a different position. In general, this system requires complex optics to correct aberrations.